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Fig. 6 Exopolysaccharides of C. somerae signal through TLR2 to inhibit SVCV infection. A Schematic representation of the study design. B–C Nonprotein macromolecule(s) mediated the antiviral effect of C. somerae. The C. somerae culture suspension was separated into cell free supernatant (CFS) and bacterial cells by centrifugation. CFS and cell lysate (CL) were separated by a 3-kDa filter, or treated with proteinase K. GF zebrafish were treated with different CFS or CL samples and subjected to SVCV infection. Viral replication was detected at 48 hpi (n = 4, pool of 30 zebrafish larvae per sample). CFS, cell free supernatant; CL, cell lysate; “-lower filtrate”, 3 kDa filtrate; “-upper retentate”, 3 kDa upper retentate; “-PK”, proteinase K treated CFS or CL samples. D Effect of morpholino-mediated TLR2 knockdown on the antiviral effect mediated by C. somerae capsular polysaccharides (CsCPS) in GF zebrafish. Viral replication was detected at 48 hpi (n = 4, pool of 30 zebrafish larvae per sample). E LPS of C. somerae does not have antiviral activity. LPS was extracted by using a commercial kit (Genmed Scientifics Inc., USA). GF zebrafish were treated with different doses of C. somerae LPS and subjected to SVCV infection. Viral replication was detected at 48 hpi (n = 4, pool of 30 zebrafish larvae per sample). Effect of morpholino-mediated TLR2 knockdown on the antiviral effect mediated by C. somerae CL (F) or CFS (G) in GF zebrafish. Viral replication was detected at 48 hpi (n = 4, pool of 30 zebrafish larvae per sample). H C. somerae exopolysaccharides (CsEPS) inhibited SVCV infection in GF zebrafish. GF zebrafish were treated with different doses of CsEPS and subjected to SVCV infection. Viral replication was detected at 48 hpi (n = 4, pool of 30 zebrafish larvae per sample). I Effect of morpholino-mediated knockdown of TLR2, Myd88, and type I IFN receptors on the antiviral effect of CsEPS in GF zebrafish. GF zebrafish were treated with CsEPS at 5 μg/mL and subjected to SVCV infection. Viral replication was detected at 48 hpi (n = 4, pool of 30 zebrafish larvae per sample). B, C, E, H One-way ANOVA followed by Dunnett’s multiple comparisons test; D, F, G, I unpaired t-test. *p < 0.05, **p < 0.01, n.s., not significant